Combined diffusion bonding and superplastic forming (DB/SPF) has recently become established as a production technique, in particular with titanium alloys. To summarise the DB/SPF process: several sheets of a superplastic metal or alloy are formed into a stack and compressed at high temperature and pressures until the sheets are diffusion bonded together. In selected locations, a stop-off material is applied to the sheets to prevent bonding in these locations while permitting bonding at the remaining locations. After diffusion bonding, and also at high temperature, the superplastic forming step is conducted that is to say an inert gas is supplied under high pressure to the interior of the stack so as to `inflate` the sheets into a three-dimensional structure. As is well known, the shape and structure of the final product is dependant on, among other things, the number of sheets in the stack and the location of the stop-off material.
In DB/SPF of titanium alloys a suitable stop-off material is fine particles of yttria in an organic resin binder, which may be applied to the sheets by conventional silk-screen printing techniques (see for example GB 2 095 137 and 1 495 655). During the DB step, which is carried out at a temperature greater than 900.degree. C. for titanium, evaporation of the binder can contaminate the surfaces of the alloy sheets; similar problems occur in the diffusion bonding of aluminum alloys. In order to produce structures, particularly those made of an aluminum base alloy, by DB/SPF techniques satisfactorily, there remains the need for a suitable stop-off material.